The present invention relates to a sleeve made of cemented carbide for casting apparatuses, and more particularly to a sleeve made of cemented carbide assembled in a low-pressure casting or die-casting apparatus at such a position that a molten metal to be cast impinges on or flows at high pressure.
The casting of low-melting point metals such as aluminum, magnesium, copper, zinc and their alloys is usually carried out by various methods using casting dies. A typical casting method is die casting by which a molten metal is supplied into a die cavity under high pressure exerted by a plunger. Another example of the casting methods is a low-pressure casting by which a molten metal is supplied into a die cavity under relatively low pressure exerted by a compressed air. In a die apparatus used for these methods, the molten metal impinges on a particular part of the apparatus or flows fast at high pressure therethrough. Such a particular part is usually called a sleeve for supplying a molten metal.
Particularly with respect to die casting apparatuses, there are two types, a hot chamber type in which a furnace for keeping the temperature of a melt high is assembled in a die machine, and a cold chamber type in which the furnace is provided separately from the die machine. The hot chamber type is suitable for low-melting point alloys of zinc, etc., and the cold chamber type is suitable for high-melting point alloys of aluminum, etc.
For instance, since an aluminum alloy is cast by a hot chamber-type die machine, an aluminum alloy melt is poured into a sleeve through its opening and then forced into a cavity under high pressure by a plunger sliding in the sleeve. A die for defining the cavity consists of two parts; one is stationally and the other is movable. After cooling the cast metal, the die is opened to remove the cast product. This die casting can easily provide castings with extremely high precision without requiring so much skills, so that it may be said to be a highly productive casting method.
The sleeve and the plunger are conventionally made of steel, such as die steel, for instance, hot die steel, SKD61 and the like.
On the other hand, used in the low-pressure casting is a die machine comprising a sleeve provided between a vertical melt supplying pipe and a die cavity. The molten metal is elevated through the vertical melt supply pipe by pressure exerted by a compressed air, and impinges on the sleeve and flows fast therethough because the inner diameter of the sleeve at its center is somewhat smaller than that of the other part of the sleeve in order to increase a melt pressure. This low-pressure casting can provide castings with substantially no pores, and since the molten metal remained in the supply pipe is recycled to the underlying furnace for reuse, its yield is very high. Likewise, the sleeve for the low-pressure casting apparatus is conventionally made of steel such as die steel, for instance, hot die steel, SKD61, and the like.
In both of them, however, a molten metal such as an aluminum melt at about 700.degree. C. or higher is poured into the sleeve through its opening or flows fast therethrough under high preasure, so that the sleeve is vulnerable to erosion by oxidation and washing with the molten metal, particularly at such positions that the molten metal impinges upon or flows faster. In particular, in a die-casting apparatus comprising a lateral sleeve, the molten metal is poured into the sleeve through an opening thereof, so that an inner wall portion of the sleeve upon which the molten metal impinges directly is highly vulnerable to erosion by oxidation and washing. The term "washing" used herein means that the inner wall of the sleeve is subjected to strong actions of a molten metal, mechanically or chemically, which may lead to severe erosion of the inner wall. One of such actions is the adhesion of a molten metal, because the removal thereof by a molten metal supplied subsequently or a plunger moving within the sleeve back and forth is likely to cause surface layer of the sleeve to come off together with the adhered metal.
After the inner wall of the sleeve is eroded, the molten metal is likely to enter into the gap or clearance between the sleeve and the plunger, causing abrasion of the inner wall of the sleeve. In addition, the intrusion of the molten metal into the gap may cause the plunger to move eccentrically, resulting in a partially enlarged clearance between the sleeve and the plunger which allows the molten metal to spew.
Attempts have been made to prevent the above phenomenon. One of them is to rotate the sleeve before the erosion takes place, so that the molten metal impinges upon a different portion of the sleeve. Another proposal is that the sleeve be divided into two or more segments so that only a segment upon which the molten metal impinges is replaced (Japanese Patent Laid-Open No. 53-70034). This proposal, however, is not successful because it is extremely difficult to form a sleeve with divided segments while ensuring smooth movement of a plunger.
Japanese Patent Laid-Open No. 52-44726 discloses a die-casting machine comprising an injection sleeve made of ceramics or cermets. The listed ceramics include Al.sub.2 O.sub.3, 3Al.sub.2 O.sub.3. 2SiO.sub.2, ZrO.sub.2, Si.sub.3 N.sub.4, etc., and the listed cermets include TiC+Mo+Ni, TiN+Co, WC+Co, ZrB.sub.2 +Mo+Ni+Fe, ZrB.sub.2 +Ti, etc. However, such ceramics do not have sufficient mechanical strength and shock resistance, and such cermets do not have sufficient resistance to oxidation and washing.
U.S. Pat. No. 3,664,411 also discloses a die-casting apparatus comprising a shot duct in which a piston moves back and forth, the shot duct being lined with a ceramic material selected to withstand contact with the metal to be cast in respect of erosion phenomena and thermal fatigue. Listed examples of such ceramic material are silicon nitride, silicon carbide and zirconia. Again, these ceramic materials are not satisfactory in terms of mechanical strength and shock resistance.
The same is true of a sleeve for the low-pressure die-casting apparatus. That is, since the sleeve has a diameter-reduced portion or a neck portion to increase the pressure of a molten metal, it is washed with the molten metal at a higher speed and pressure. Thus, it is similarly subjected to oxidation and washing with the molten metal such as aluminum. The use of the eroded sleeve would cause uneven flow of the molten metal which in turn causes pinholes, cracks, blemishes, disfigurements, etc. on the cast products.